Infectious cDNA clones of echovirus 12 and a variant resistant against the uncoating inhibitor rhodanine differ in seven amino acids.

Determination of the complete sequences of echovirus 12 and a rhodanine-resistant variant revealed seven amino acid deviations and two additional exchanges not confirmed in all clones. In rhodanine sensitivity assays with infectious cDNAs, it was shown that the biological markers of the original viruses are maintained.     

Allele-specific adaptation of poliovirus VP1 B-C loop variants to mutant cell receptors.

Previous work has shown that three different mutations in domain 1 of the poliovirus receptor (Pvr), two in the predicted C'-C" ridge and one in the D-E loop, abolish binding of the P1/Mahoney strain. All three receptor defects could be suppressed by a mutation in the VP1 B-C loop of the viral capsid that was present in all 16 P1/Mahoney isolates adapted to the mutant receptors. To identify allele-specific mutations that enable poliovirus to utilize mutant receptors, and to understand the role of the VP1 B-C loop in adaptation, we selected mutant receptor-adapted viruses derived from two P1/Mahoney variants, one which lacks the VP1 B-C loop and one in which the VP1 B-C loop is replaced with the corresponding sequence from the P2/Lansing strain. Six adapted viral isolates were obtained after passage on mutant receptor-expressing cell lines. Sequence analysis revealed that each virus contained three to five mutations, and a total of 18 amino acid changes at 17 capsid residues were identified. Site-directed mutagenesis was used to evaluate the role of these mutations in adaptation to mutant Pvr. The results demonstrate that mutations in the viral canyon floor and rim are allele specific and compensate only for receptor defects in the C'-C" ridge of Pvr, suggesting that these sites interact in the virus-receptor complex. Furthermore, mutations in the VP1 E-F loop suppressed Pvr D-E loop defects, implying that the Pvr D-E loop contacts the VP1 E-F loop. Most of the other mutations mapped to interior capsid residues, some interacting with the fivefold- or threefold-related protomers. These mutations may regulate receptor interaction by controlling the structural flexibility of the viral capsid. In viruses lacking the VP1 B-C loop, single mutations were not sufficient to confer the adapted phenotype, in contrast to the 414 virus, which contains the B-C loop. Although the VP1 B-C loop appeared to be dispensable for adaptation, it may have provided a selective advantage in adaptation of P1/Mahoney to mutant Pvr.     

Antiviral activity of pyridyl imidazolidinones against enterovirus 71 variants

Pyridyl imidazolidinone is a novel class of capsid binder which can inhibit enterovirus 71 (EV71). In this study, we tested the susceptibility of six recombinant viruses with different single-site mutations in VP1. Eleven modified pyridyl imidazolidinones were synthesized and used to probe the interaction between these compounds and the EV71 VP1 protein. We found that the D31N or E98K mutant viruses were susceptible to bulkier compounds, which suggested that mutations at these two sites in VP1 may widen the hydrophobic pocket of VP1, allowing bulkier compounds to enter and interfere VP1-receptor binding. Additionally, the Y116H mutant was more resistant to pyridyl imidazolidinone compounds containing a methyl group in the central position of the hydrophobic linker. When a trifluoromethyl group was substituted for the methyl group in the middle of the linker chain, the inhibitory effect was totally abolished in the Y116H mutant, suggesting that the interaction between Tyr (Y) 116 of VP1 and the central position of the linker chain of pyridyl imidazolodinone is very important for drug efficacy. A V192M mutant was resistant to most of the derivatives, indicating that residue 192 is a key mutation for resistance to pyridyl imidazolidinone.     

Host range determinants located on the interior of the poliovirus capsid.

The inability of certain poliovirus strains to infect mice can be overcome by the expression of human poliovirus receptors in mice or by the presence of a particular amino acid sequence of the B-C loop of the viral capsid protein VP1. We have identified changes in an additional capsid structure that permit host-restricted poliovirus strains to infect mice. Variants of the mouse-virulent P2/Lansing strain were constructed containing amino acid changes, deletions and insertions in the B-C loop of VP1. These variants were attenuated in mice, demonstrating the importance of the B-C loop sequence in host range. Passage of two of the B-C loop variants in mice led to the selection of viruses that were substantially more virulent. The increased neurovirulence of these strains was mapped to two different suppressor mutations in the N-terminus of VP1. Whereas the B-C loop of VP1 is highly exposed on the surface of the capsid, near the five-fold axis of symmetry, the suppressor mutations are in the interior of the virion, near the three-fold axis. Introduction of the suppressor mutations into the genome of the mouse-avirulent P1/Mahoney strain resulted in neurovirulent viruses, demonstrating that the P2/Lansing B-C loop sequence is not required to infect mice. Because the internal host range determinants are in a structure known to be important in conformational transitions of the virion, the host range of poliovirus may be determined by the ability of virions to undergo transitions catalyzed by cell receptors.     

Identification of a region of the poliovirus genome involved in persistent infection of HEp-2 cells.

Poliovirus mutants were selected during the persistent infection of human neuroblastoma cells. These viruses could establish secondary persistent infections in HEp-2 nonneural cells. We report the identification of a region of the genome of a persistent virus (S11) that was sufficient to confer to a recombinant virus the phenotype that causes persistent infection in HEp-2 cells. This region, between nucleotides 1148 and 3481, contained 11 missense mutations mapping exclusively in the genes of capsid proteins VP1 and VP2. Because recombinant viruses carrying only one of these two mutated genes were not able to cause persistent infection, it seems very probable that two or more mutations in these genes are required for expression of the phenotype that causes persistent infection.     

A domain in the herpes simplex virus 1 triplex protein VP23 is essential for closure of capsid shells into icosahedral structures

VP23 is a key component of the triplex structure. The triplex, which is unique to herpesviruses, is a complex of three proteins, two molecules of VP23 which interact with a single molecule of VP19C. This structure is important for shell accretion and stability of the protein coat. Previous studies utilized a random transposition mutagenesis approach to identify functional domains of the triplex proteins. In this study, we expand on those findings to determine the key amino acids of VP23 that are required for triplex formation. Using alanine-scanning mutagenesis, we have made mutations in 79 of 318 residues of the VP23 polypeptide. These mutations were screened for function both in the yeast two-hybrid assay for interaction with VP19C and in a genetic complementation assay for the ability to support the replication of a VP23 null mutant virus. These assays identified a number of amino acids that, when altered, abolish VP23 function. Abrogation of virus assembly by a single-amino-acid change bodes well for future development of small-molecule inhibitors of this process. In addition, a number of mutations which localized to a C-terminal region of VP23 (amino acids 205 to 241) were still able to interact with VP19C but were lethal for virus replication when introduced into the herpes simplex virus 1 (HSV-1) KOS genome. The phenotype of many of these mutant viruses was the accumulation of large open capsid shells. This is the first demonstration of capsid shell accumulation in the presence of a lethal VP23 mutation. These data thus identify a new domain of VP23 that is required for or regulates capsid shell closure during virus assembly.     

Amino acid substitutions in the poliovirus maturation cleavage site affect assembly and result in accumulation of provirions.

The assembly of infectious poliovirus virions requires a proteolytic cleavage between an asparagine-serine amino acid pair (the maturation cleavage site) in VP0 after encapsidation of the genomic RNA. In this study, we have investigated the effects that mutations in the maturation cleavage site have on P1 polyprotein processing, assembly of subviral intermediates, and encapsidation of the viral genomic RNA. We have made mutations in the maturation cleavage site which change the asparagine-serine amino acid pair to either glutamine-glycine or threonine-serine. The mutations were created by site-directed mutagenesis of P1 cDNAs which were recombined into wild-type vaccinia virus to generate recombinant vaccinia viruses. The P1 polyproteins expressed from the recombinant vaccinia viruses were analyzed for proteolytic processing and assembly defects in cells coinfected with a recombinant vaccinia virus (VV-P3) that expresses the poliovirus 3CD protease. A trans complementation system using a defective poliovirus genome was utilized to assess the capacity of the mutant P1 proteins to encapsidate genomic RNA (D. C. Ansardi, D. C. Porter, and C. D. Morrow, J. Virol. 67:3684-3690, 1993). The mutant P1 proteins containing the glutamine-glycine amino acid pair (VP4-QG) and the threonine-serine pair (VP4-TS) were processed by 3CD provided in trans from VV-P3. The processed capsid proteins VP0, VP3, and VP1 derived from the mutant precursor VP4-QG were unstable and failed to assemble into subviral structures in cells coinfected with VV-P3. However, the capsid proteins derived from VP4-QG did assemble into empty-capsid-like structures in the presence of the defective poliovirus genome. In contrast, the capsid proteins derived from processing of the VP4-TS mutant assembled into subviral intermediates both in the presence and in the absence of the defective genome RNA. By a sedimentation analysis, we determined that the capsid proteins derived from the VP4-TS precursor encapsidated the defective genome RNA. However, the cleavage of VP0 to VP4 and VP2 was delayed, resulting in the accumulation of provirions. The maturation cleavage of the VP0 protein containing the VP4-TS mutation was accelerated by incubation of the provirions at 37 degrees C. The results of these studies demonstrate that mutations in the maturation cleavage site have profound effects on the subsequent capability of the capsid proteins to assemble and provide evidence for the existence of the provirion as an assembly intermediate.     

Distribution of drug resistance mutations in type 3 poliovirus identifies three regions involved in uncoating functions.

We have previously described the use of an uncoating inhibitor, WIN 51711, to select drug-resistant mutants of the Sabin strain of poliovirus type 3. Two-thirds of the mutants proved to be dependent on the drug for plaque formation because of extreme thermolability (A. G. Mosser and R. R. Rueckert, J. Virol. 67:1246-1254, 1993). Here we report the responsible mutations; all were traced to single amino acid substitutions. Mutations conferring dependence and thermolability occurred in all four capsid proteins (VP1 to VP4), but all were clustered near residue 53 of VP4 at the inner capsid surface. Amino acid substitutions of the remaining non-drug-dependent mutants were mapped to three distinct loci: (i) on or near the inner capsid surface, at VP4 residue 46 or VP1 residue 129, in the vicinity of the drug dependence substitutions; (ii) at residues 192, 194, and 260 in the lining of the VP1 beta barrel, which is the drug-binding site; and (iii) at VP1 residue 105 on the edge of the canyon surrounding the fivefold axis of symmetry, the putative receptor-binding site. All of the mutations increased the eclipse rate of cell-attached virus. Such mutants help identify parts of the capsid that play a role in viral uncoating functions.     

Reverse Genetic Analysis of Adaptive Mutations within the Capsid Proteins of Enterovirus 71 (EV-A71) Strains Necessary for Infection of CHO-K1 Cells

正Dear Editor,Previous studies had described the adaptation of enterovirus 71 (EV-A71) strains that enabled entry and viral replication in Chinese Hamster Ovary (CHO) cell line(Zaini and Mc Minn 2012; Zaini et al. 2012). These adapted     

Functional analysis of the triplex proteins (VP19C and VP23) of herpes simplex virus type 1

The triplex of herpesvirus capsids is a unique structural element. In herpes simplex virus type 1 (HSV-1), one molecule of VP19C and two of VP23 form a three-pronged structure that acts to stabilize the capsid shell through interactions with adjacent VP5 molecules. The interaction between VP19C and VP23 was inferred by yeast cryoelectron microscopy studies and subsequently confirmed by the two-hybrid assay. In order to define the functional domains of VP19C and VP23, a Tn7-based transposon was used to randomly insert 15 bp into the coding regions of these two proteins. The mutants were initially screened for interaction in the yeast two-hybrid assay to identify the domains important for triplex formation. Using genetic complementation assays in HSV-1-infected cells, the domains of each protein required for virus replication were similarly uncovered. The same mutations that abolish interaction between these two proteins in the yeast two-hybrid assay similarly failed to complement the growth of the VP23- and VP19C-null mutant viruses in the genetic complementation assay. Some of these mutants were transferred into recombinant baculoviruses to analyze the effect of the mutations on herpesvirus capsid assembly in insect cells. The mutations that abolished the interaction in the yeast two-hybrid assay also abolished capsid assembly in insect cells. The outcome of these experiments showed that insertions in at least four regions and especially the amino terminus of VP23 abolished function, whereas the amino terminus of VP19C can tolerate transposon insertions. A novel finding of these studies was the ability to assemble herpesvirus capsids in insect cells using VP5 and VP19C that contained a histidine handle at their amino terminus.     

Residues of VP26 of herpes simplex virus type 1 that are required for its interaction with capsids

VP26 is the smallest capsid protein and decorates the outer surface of the capsid shell of herpes simplex virus. It is located on the hexons at equimolar amounts with VP5. Its small size (112 amino acids) and high copy number make it an attractive molecule to use as a probe to investigate the complex pattern of capsid protein interactions. An in vitro capsid binding assay and a green fluorescent protein (GFP) localization assay were used to identify VP26 residues important for its interaction with capsids. To test for regions of VP26 that may be essential for binding to capsids, three small in-frame deletion mutations were generated in VP26, Delta18-25, Delta54-60, and Delta93-100. Their designations refer to the amino acids deleted by the mutation. The mutation at the C terminus of the molecule, which encompasses a region of highly conserved residues, abolished binding to the capsid and the localization of GFP to the nucleus in characteristic large puncta. Additional mutations revealed that a region of VP26 spanning from residue 50 to 112 was sufficient for the localization of the fused protein (VP26-GFP) to the nucleus and for it to bind to capsids. Using site-directed mutagenesis of conserved residues in VP26, two key residues for protein-protein interaction, F79 and G93, were identified as judged by the localization of GFP to nuclear puncta. When these mutations were analyzed in the capsid binding assay, they were also found to eliminate binding of VP26 to the capsid structure. Surprisingly, additional mutations that affected the ability of VP26 to bind to capsids in vitro were uncovered. Mutations at residues A58 and L64 resulted in a reduced ability of VP26 to bind to capsids. Mutation of the hydrophobic residues M78 and A80, which are adjacent to the hydrophobic residue F79, abolished VP26 capsid binding. In addition, the block of conserved amino acids in the carboxy end of the molecule had the most profound effect on the ability of VP26 to interact with capsids. Mutation of amino acid G93, L94, R95, R96, or T97 resulted in a greatly diminished ability of VP26 to bind capsids. Yet, all of these residues other than G93 were able to efficiently translocate or concentrate GFP into the nucleus, giving rise to the punctate fluorescence. Thus, the interaction of VP26 with the capsid appears to occur through at least two separate mechanisms. The initial interaction of VP26 and VP5 may occur in the cytoplasm or when VP5 is localized in the nucleus. Residues F79 and G93 are important for this bi-molecular interaction, resulting in the accumulation of VP26 in the nucleus in concentrated foci. Subsequent to this association, additional amino acids of VP26, including those in the C-terminal conserved domain, are important for interaction of VP26 with the three-dimensional capsid structure.     

Identification of an essential domain in the herpesvirus VP1/2 tegument protein: the carboxy terminus directs incorporation into capsid assemblons

The herpesvirus tegument is a layer of viral and cellular proteins located between the capsid and envelope of the virion. The VP1/2 tegument protein is critical for the propagation of all herpesviruses examined. Using an infectious clone of the alphaherpesvirus pseudorabies virus, we have made a collection of truncation and in-frame deletion mutations within the VP1/2 gene (UL36) and examined the resulting viruses for spread between cells. We found that the majority of the VP1/2 protein either was essential for virus propagation or did not tolerate large deletions. A recently described amino-terminal deubiquitinase-encoding domain was dispensable for alphaherpesvirus propagation, but the rate of propagation in an epithelial cell line and the frequency of transport in axons of primary sensory neurons were both reduced. We mapped one essential domain to a conserved sequence at the VP1/2 carboxy terminus and demonstrated that this domain sufficient to redirect the green fluorescent protein to capsid assemblons in nuclei of infected cells.     

The recognition of parvovirus capsids by antibodies

The parvoviruses are small, non-enveloped icosahedral viruses which infect many animals, including vertebrates and arthropods. Vertebrate parvoviruses can be classified into the autonomous and the adeno-associated viruses — the autonomous parvoviruses have been examined in detail for antigenic structure. The protective immunity against parvoviruses in animals appears to be primarily antibody-mediated. The capsid of the autonomous parvoviruses is assembled from two proteins, VP1 and VP2, which overlap in sequence, with VP1 having additional N-terminal residues. Empty capsids can be assembled from VP2 alone.The structures of canine parvovirus (CPV) and feline panleukopenia virus (FPV) have been solved to better than 3·5 Å resolution, and the structure of human parvovirus, B19, has been solved to 8 Å resolution. In each case the T = 1 icosahedron is made up to 60 copies of a structural motif common to VP1 and VP2, consisting of an eight-stranded anti-parallel β-barrel. The surface of the capsid is made up primarily of large elaborate loops which connect the β-strands that make up the barrel. Antigenic epitopes have been mapped utilizing escape mutants, natural variants, peptide analysis and by expression of viral proteins. In CPV two major antigenic determinants were defined by escape mutant analysis, while peptide analysis revealed antigenic determinants in many different regions of the capsid protein, including the amino terminus of VP2. Neutralizing epitopes of B19 were found by peptide analysis in the VP1-unique region and in sequences common to VP1 and VP2. Other antigenic, but non-neutralizing, epitopes were found in the VP1–VP2 junction, as well as various other parts of the VP2 protein.The binding of a Fab derived from one neutralizing anti-CPV Mab has been examined by cryo-electron microscopy image reconstruction, which showed that 60 copies of the Fab were bound per virion. The Fab footprint covered approximately 796 Ųof the capsid surface, in a region where escape mutations to that Mab had been previously shown to cluster. The mechanism of neutralization was not clear, but could involve interference with cell attachment, cell entry or uncoating during the process of cell infection.     

The Herpesvirus capsid surface protein, VP26, and the majority of the tegument proteins are dispensable for capsid transport toward the nucleus

Upon entering a cell, alphaherpesvirus capsids are transported toward the minus ends of microtubules and ultimately deposit virus DNA within the host nucleus. The virus proteins that mediate this centripetal transport are unknown but are expected to be either viral tegument proteins, which are a group of capsid-associated proteins, or a surface component of the capsid itself. Starting with derivatives of pseudorabies virus that encode a fluorescent protein fused to a structural component of the virus, we have made a collection of 12 mutant viruses that lack either the VP26 capsid protein or an individual tegument protein. Using live-cell fluorescence microscopy, we tracked individual virus particles in axons following infection of primary sensory neurons. Quantitative analysis of the VP26-null virus indicates that this protein plays no observable role in capsid transport. Furthermore, viruses lacking tegument proteins that are nonessential for virus propagation in cell culture were also competent for axonal transport. These results indicate that a protein essential for viral propagation mediates transport of the capsid to the nucleus.     

Characterization of bovine rotavirus VP6 and VP7 as glycoproteins using monoclonal antibodies

Bovine rotavirus proteins were analysed by a panel of monoclonal antibodies. Glycosylated epitopes were identified on both inner and outer capsid proteins (VP6 and VP7 respectively). VP7 possessed a periodate insensitive epitope which was, however, sensitive to endoglycosidase H, mixed glycosidases and to protease treatment. This epitope was not detected on viruses grown in the presence of 2-deoxy-D-glucose or tunicamycin. An epitope was detected on VP6 which was sensitive to periodate oxidation. The blotted protein reacted with a glycan assay kit; yet the epitope was not affected by endoglycosidase H and was found on viruses grown in the presence of 2-deoxy-D-glucose or tunicamycin. These results suggest that VP7 and VP6 epitopes are carbohydrate dependent. The VP7 epitope contains an N-linked carbohydrate moiety in contrast to the VP6 epitope which appears to contain O-linked glycosyl units.     

Mutation of single hydrophobic residue I27, L35, F39, L58, L65, L67, or L71 in the N terminus of VP5 abolishes interaction with the scaffold protein and prevents closure of herpes simplex virus type 1 capsid shells

Protein-protein interactions drive the assembly of the herpes simplex virus type 1 (HSV-1) capsid. A key interaction occurs between the C-terminal tail of the scaffold protein (pre-22a) and the major capsid protein (VP5). Previously (Z. Hong, M. Beaudet-Miller, J. Durkin, R. Zhang, and A. D. Kwong, J. Virol. 70:533-540, 1996) it was shown that the minimal domain in the scaffold protein necessary for this interaction was composed of a hydrophobic amphipathic helix. The goal of this study was to identify the hydrophobic residues in VP5 important for this bimolecular interaction. Results from the genetic analysis of second-site revertant virus mutants identified the importance of the N terminus of VP5 for the interaction with the scaffold protein. This allowed us to focus our efforts on a small region of this large polypeptide. Twenty-four hydrophobic residues, starting at L23 and ending at F84, were mutated to alanine. All the mutants were first screened for interaction with pre-22a in the yeast two-hybrid assay. From this in vitro assay, seven residues, I27, L35, F39, L58, L65, L67, and L71, that eliminated the interaction when mutated were identified. All 24 mutants were introduced into the virus genome with a genetic marker rescue/marker transfer system. For this system, viruses and cell lines that greatly facilitated the introduction of the mutants into the genome were made. The same seven mutants that abolished interaction of VP5 with pre-22a resulted in an absolute requirement for wild-type VP5 for growth of the viruses. The viruses encoding these mutations in VP5 were capable of forming capsid shells comprised of VP5, VP19C, VP23, and VP26, but the closure of these shells into an icosahedral structure was prevented. Mutation at L75 did not affect the ability of this protein to interact with pre-22a, as judged from the in vitro assay, but this mutation specified a lethal effect for virus growth and abolished the formation of any detectable assembled structure. Thus, it appears that the L75 residue is important for another essential interaction of VP5 with the capsid shell proteins. The congruence of the data from the previous and present studies demonstrates the key roles of two regions in the N terminus of this large protein that are crucial for this bimolecular interaction. Thus, residues I27, L35, and F39 comprise the first subdomain and residues L58, L65, L67 and L71 comprise a second subdomain of VP5. These seven hydrophobic residues are important for the interaction of VP5 with the scaffold protein and consequently the formation of an icosahedral shell structure that encloses the viral genome.     

Coxsackievirus A9 VP1 mutants with enhanced or hindered A particle formation and decreased infectivity

We have studied coxsackievirus A9 (CAV9) mutants that each have a single amino acid substitution in the conserved 29-PALTAVETGHT-39 motif of VP1 and a reduced capacity to produce infectious progeny virus. After uncoating, all steps in the infection cycle occurred according to the same kinetics as and similar efficiency to the wild-type virus. However, the particle/infectious unit ratio in the progeny was significantly increased. The differences were apparently due to altered stability of the capsid: there were mutant viruses with enhanced or hindered uncoating, and both of these characteristics were found to reduce fitness under standard passaging conditions. At 32 degrees C the instable mutants had an advantage, while the wild-type and the most stable mutant grew poorly. When comparing the newly published CAV9 structure and the other enterovirus structures, we found that the PALTAVETGHT motif is always in exactly the same position, in a cavity formed by the 3 other capsid proteins, with the C terminus of VP4 between this motif and the RNA. In the 7 enterovirus structures determined to date, the most conserved residues of the studied motif have identical contacts to neighboring residues of VP2, VP3, and VP4. We conclude that (i) the mutations affect the uncoating step necessary for infection, resulting in an untimely or hindered externalization of the VP1 N terminus together with the VP4, and (ii) the reason for the studied motif being evolutionarily conserved is its role in maintaining an optimal balance between the protective stability and the functional flexibility of the capsid.     

Sindbis virus vectors designed to express a foreign protein as a cleavable component of the viral structural polyprotein

Alphavirus-based expression vectors commonly use a duplicated 26S promoter to drive expression of a foreign gene. Here we describe an expression strategy in which the foreign sequences are linked to the gene encoding the 2A protease of foot-and-mouth disease virus and then inserted in frame between the capsid and E3 genes of Sindbis virus. During replication, the 2A fusion protein is synthesized as a component of the viral structural polyprotein that is then released by intramolecular cleavages mediated by the capsid and 2A proteases. Recombinant Sindbis viruses that expressed fusion proteins composed of 2A linked to the green fluorescent protein (GFP) and to the VP7 protein of bluetongue virus were constructed. Viruses engineered to express GFP and VP7 from a duplicate 26S promoter were also constructed. All four viruses expressed the transgene and grew to similar titers in cultured cells. However, the GFP/2A- and VP7/2A-expressing viruses displayed greater expression stability and were less attenuated in newborn mice than the cognate double-subgenomic promoter-based viruses. By combining the two expression strategies, we constructed bivalent viruses that incorporated and expressed both transgenes. The bivalent viruses grew to lower titers in cultured cells and were essentially avirulent in newborn mice. Groups of mice were vaccinated with each VP7- and VP7/2A-expressing virus, and antibody responses to native VP7 were measured in an indirect enzyme-linked immunosorbent assay. Despite their genetic and phenotypic differences, all viruses induced similarly high titers of VP7-specific antibodies. These results demonstrate that 2A fusion protein-expressing alphaviruses may be particularly well suited for applications that require enduring expression of a single protein or coexpression of two alternative proteins.     

Evolution of the capsid protein genes of foot-and-mouth disease virus: antigenic variation without accumulation of amino acid substitutions over six decades.

The genetic diversification of foot-and-mouth disease virus (FMDV) of serotype C over a 6-decade period was studied by comparing nucleotide sequences of the capsid protein-coding regions of viruses isolated in Europe, South America, and The Philippines. Phylogenetic trees were derived for VP1 and P1 (VP1, VP2, VP3, and VP4) RNAs by using the least-squares method. Confidence intervals of the derived phylogeny (significance levels of nodes and standard deviations of branch lengths) were placed by application of the bootstrap resampling method. These procedures defined six highly significant major evolutionary lineages and a complex network of sublines for the isolates from South America. In contrast, European isolates are considerably more homogeneous, probably because of the vaccine origin of several of them. The phylogenetic analysis suggests that FMDV CGC Ger/26 (one of the earliest FMDV isolates available) belonged to an evolutionary line which is now apparently extinct. Attempts to date the origin (ancestor) of the FMDVs analyzed met with considerable uncertainty, mainly owing to the stasis noted in European viruses. Remarkably, the evolution of the capsid genes of FMDV was essentially associated with linear accumulation of silent mutations but continuous accumulation of amino acid substitutions was not observed. Thus, the antigenic variation attained by FMDV type C over 6 decades was due to fluctuations among limited combinations of amino acid residues without net accumulation of amino acid replacements over time.     

Epitope insertion at the N-terminal molecular switch of the rabbit hemorrhagic disease virus T = 3 capsid protein leads to larger T = 4 capsids

Viruses need only one or a few structural capsid proteins to build an infectious particle. This is possible through the extensive use of symmetry and the conformational polymorphism of the structural proteins. Using virus-like particles (VLP) from rabbit hemorrhagic disease virus (RHDV) as a model, we addressed the basis of calicivirus capsid assembly and their application in vaccine design. The RHDV capsid is based on a T=3 lattice containing 180 identical subunits (VP1). We determined the structure of RHDV VLP to 8.0-Å resolution by three-dimensional cryoelectron microscopy; in addition, we used San Miguel sea lion virus (SMSV) and feline calicivirus (FCV) capsid subunit structures to establish the backbone structure of VP1 by homology modeling and flexible docking analysis. Based on the three-domain VP1 model, several insertion mutants were designed to validate the VP1 pseudoatomic model, and foreign epitopes were placed at the N- or C-terminal end, as well as in an exposed loop on the capsid surface. We selected a set of T and B cell epitopes of various lengths derived from viral and eukaryotic origins. Structural analysis of these chimeric capsids further validates the VP1 model to design new chimeras. Whereas most insertions are well tolerated, VP1 with an FCV capsid protein-neutralizing epitope at the N terminus assembled into mixtures of T=3 and larger T=4 capsids. The calicivirus capsid protein, and perhaps that of many other viruses, thus can encode polymorphism modulators that are not anticipated from the plane sequence, with important implications for understanding virus assembly and evolution.